With respect to superconductors such as superconducting wires, increasing the thickness of a superconducting layer has been studied in order to increase the critical current.
However, if a superconducting layer is formed so as to have a large thickness by deposition at one time, a sufficient amount of oxygen cannot be taken in the superconducting layer during the film deposition. In such a case, the critical current density (hereinafter referred to as “Jc”) is decreased, and accordingly the Ic cannot be increased. Furthermore, if the superconducting layer is formed so as to have a large thickness by performing the film deposition once, the time required for one film deposition process becomes long. As a result, an element, such as Ni, in a base layer is diffused into the superconducting layer, resulting in a problem such as reaction with the superconducting layer.
Therefore, in order to form a thick superconducting layer and to increase the Ic, a multi-layer deposition method has been studied, in which the film deposition is performed at least two times and the thickness of a superconducting film in each film deposition is decreased (For example, see Kazutomi Kakimoto and two others, “Preparation of Y-system coated conductor with high Ic using repeated deposition by PLD”, Abstracts of the 67th Cryogenic Association of Japan (CSJ) Conference in 2002, p. 228).
According to Kakimoto et al., the thickness of the superconducting film was controlled to be 0.35 μm in each film deposition and the deposition was performed 4 to 6 times. Even when the thickness of the superconducting layer (10 mm in width) was 1 μm or more, the Ic was as small as about 130 A. In the experiment described in the above-mentioned paper, the moving speed of a base tape having a width of 10 mm on which the superconducting layer was formed was 4 m/h.
Accordingly, it is desired that further increase in the Ic of superconductors be achieved.